We use the idealized GCM described by [1]} and [2]}, which is based on the spectral dynamical core of the Flexible Modeling System developed at the Geophysical
Fluid Dynamics Laboratory. To approximate the solution of the hydrostatic primitive equations, it uses the spectral transform method in the horizontal, with spectral resolution T21 and with 32 latitude points on the transform grid. It uses finite differences with 10 unevenly spaced sigma levels in the vertical. We chose this relatively coarse resolution to keep our numerical
experiments computationally efficient, so that comparison of CES with much more expensive methods is feasible. The lower boundary of the GCM is a homogeneous slab ocean (1 m mixed-layer thickness). Radiative transfer is represented by a semi-gray, two-stream radiative transfer scheme, in which the optical depth of longwave and shortwave absorbers is a prescribed function of latitude and pressure [2]}, irrespective of the concentration of water vapor in the atmosphere (i.e., without an explicit representation of water vapor feedback). Insolation is constant and approximates Earth's annual mean insolation at the top of the atmosphere.
